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A tearful Christie Brinkley testified at her divorce trial Thursday that she was devastated by her husband’s affair with a teenager and his attempts to find other paramours through pornographic Web sites.

“My world was completely shattered,” the former Sports Illustrated model said. “My life as I knew it had vanished.”

Brinkley took the stand in state Supreme Court a day after the trial opened with testimony from her architect-husband, Peter Cook, and his former mistress, Diana Bianchi, who was 18 when she began seeing him. Brinkley and Cook are mainly arguing over custody of their children, ages 10 and 13.

Brinkley said she learned about the betrayal when Bianchi’s stepfather tapped her on the shoulder after she spoke at a Southampton High School graduation in June 2006.

“That husband of yours won’t knock it off. He’s having an affair with my teenage daughter,” said the stepfather, Brian Platt, a Southampton

Village police officer who testified to the same thing Wednesday.

Brinkley said she looked at Cook, sitting in the front row at the graduation. When he shook his head in denial before she even confronted him with the allegation, she thought: “My God, it’s true. He did do that.”

Cook initially denied the liaison, but he has since apologised and argues it should not disqualify him from getting custody of his children. He testified earlier this week that he gave Bianchi a $300,000 payoff, had trysts with her in his office and Brinkley’s Hamptons homes and spent thousands of dollars on online porn.

Brinkley said after learning of his cheating she told him to give his computer passwords to her “if you’ve got nothing to hide” and found e-mails from Bianchi and porn sites he had visited. He was trying “to connect with people” in their neighborhood, she said.

“It was not a voyeur site. It was a meeting site,” she said. “It was more than I could bear.”

Cook, who testified that Bianchi was his only extramarital lover, said he and Brinkley used pornography “to get the mood going” during the last half of their 10-year marriage.

Brinkley said that before she learned of the affair, the marriage’s biggest problem was Cook’s relationship with Alexa Ray Joel, her daughter with “Piano Man” singer Billy Joel.

“You married me and my children,” Brinkley said she would tell Cook. “You’ve got to find a way to love Alexa.”

Alexa Joel, 22, testified that Cook treated her well before he married her mother but criticized her constantly afterward. She said he called her out for chewing loudly, complained about her piano playing and once shoved her head into a bucket of water while demanding she clean up a plumbing leak caused by her lengthy shower.

Cook denied Joel’s account when he returned to the stand Thursday, saying he had “absolutely not” pushed her into the bucket. He said he tried to stop her “loud, aggressive” piano playing only when his and Brinkley’s younger children were going to bed and was only backing up Brinkley’s own admonishments about her daughter’s chewing and protracted showers.